L2TP, or Layer 2 Tunneling Protocol, is a tunneling protocol that (not surprisingly) operates on the Link layer of the seven-layer OSI model. Unlike IPsec, it does not provide any encryption or confidentiality by itself, but instead relies on whatever encryption protocol is passing through its tunnel. As a result, it is often used in conjunction with IPsec.
The client end of an L2TP tunnel is known as the L2TP Access Concentrator (LAC). The server end is known as the L2TP Network Server (LNS). The LNS end, once configured, waits for new connections. A connection is established through the exchange of several control packets, for which L2TP provides reliability. No reliability is provided for data packets, although reliability may be ...
